In general, the electronic device, etc. mounted to the vehicle are operated under an environment tending to be extremely changed in temperature. Therefore, it becomes an important subject that the electronic device, etc. are stably operated by excluding an influence of temperature in accordance with a function of the electronic device. When protection against an overcurrent is performed with respect to the semiconductor element for a driver for outputting a communication signal with respect to the electronic device to the communication line, it is also desirable to stably perform this protecting operation without being influenced by temperature.
For example, Japanese Patent Application Publication No. 2003-218673 discloses a construction for detecting a consumed electric current of a sensor by using a shunt resistor in a circuit to perform overload and short circuit detection with respect to a slave for an actuator sensor interface circuit.
Further, U.S. Pat. No. 4,553,084 discloses a construction in which a transistor for mainly flowing a load electric current and a transistor for detecting the electric current by the shunt resistor are connected in parallel and are constructed on one chip, and a cell ratio of the transistors is set to a suitable ratio so that the detection is performed by flowing a microscopic electric current onto the shunt resistor side.
However, no temperature characteristics provided in the shunt resistor are considered in a technique disclosed in Japanese Patent Application Publication No. 2003-218673. In addition, this technique is set to a construction for judging a voltage drop caused in this shunt resistor by utilizing the voltage between a base and an emitter of the transistor. Therefore, it is easily influenced by temperature. As its result, a problem exists in that a protecting electric current value greatly depends on operation temperature.
A technique disclosed in U.S. Pat. No. 4,553,084 is suitable for a case for using a power transistor as in the treatment of a large electric current. However, more transistors are required. When it is supposed that this technique is applied to the protection of the semiconductor element for a driver for outputting the communication signal to the electronic device mounted to the vehicle, a transistor for adjusting the electric current is excessively added so that the construction becomes complicated. Further, since the shunt resistor is used in the electric current detection, there is a problem similar to that of Japanese Patent Application Publication No. 2003-218673.